In the field of color printing, primary colors are used together in a manner that results in intermediate colors. For example, an ink jet mechanism may have cyan, magenta, yellow and black print heads. In response, a digital file format associated with a color document may have data related to three color planes, i.e. cyan, magenta and yellow. By using these three planes, additional colors may be produced. For example, two or more color planes may be halftoned together to result in a region of print having a desired color. In particular, cyan and magenta are frequently halftoned to result in a spectrum of colors between these primary colors.
The color planes in the digital file format may, for example, result in an unbalanced output dot pattern that is approximately 80% to 20% each of cyan and magenta dots. Alternatively, the color planes may result in a balanced dot pattern, such as 55% cyan and 45% magenta. In both of these cases, i.e. unbalanced and balanced outputs, it is not necessary for the output to total 100%. Where the planes total less than 100%, some of the white of the paper will show through, resulting in a softer, pastel-like, shade.
Within certain regions of the document, the sum of the color planes may exceed 100%, thereby resulting in pixel locations wherein two or more colors of ink are applied. For example, in those locations within a document wherein the sum of the dot patterns of the cyan and magenta planes is over 100%, cyan and magenta dots will be applied on top of each other at a given pixel location, resulting in blue output. In this case, the color planes may be viewed as overlapping.
Within regions wherein the dot patterns from the cyan and magenta planes are unbalanced, the print output may have the appearance, particularly at close range, of short chains or lines of the minority dot color within a background comprising the majority color. Similarly, within regions wherein the dot patterns from the cyan and magenta planes are balanced, the print output may have the appearance, particularly at close range, of vertical stripes that alternate between cyan and magenta. And still further, within regions wherein the dot patterns from the cyan and magenta planes are overlapping, the print output may have the appearance, particularly at close range, of non-homogeneously distributed blue dots within regions of color derived from combination of the cyan and magenta color planes. All three of these conditions tend to catch the human eye, and suggest poor print quality.
FIG. 1, prior art, shows an enlarged view of the dot pattern of a conventional halftoning algorithm. A printed document 100 includes a plurality of regions 102 wherein a single color is intended. A first enlarged view 104 illustrates the output of unbalanced cyan and magenta color planes. Individual pixels 106 represent locations onto which dots of ink may be printed, where pixels designated with a “c” represent cyan and pixels designated with an “m,” represent magenta. These pixels are organized into a pattern having minority cyan pixels grouped or bunched together within a larger region of the majority magenta pixels. This output will not have an aesthetic appearance, and in particular will be spotted and non-homogeneous.
Similarly, a second enlarged view 108 illustrates the output of balanced color planes. In this case, the cyan and magenta pixels are organized in vertical stripes, horizontal stripes, checkerboards or some combination of these. Such grouping is generally unaesthetic, and suggests some failure of the print mechanism. Additionally, in a third enlarged view 110, blue pixels (represented by “b”) distributed unattractively in groups within a background of cyan.
The problems seen in FIG. 1 frequently result in the translation of digital file formats. For example, an 8-bit RGB (red green blue) file format is adapted for display on a cathode ray tube (or similar) video display monitor. To print such a file, it must be translated from RGB to a 1-bit CMYK (cyan magenta yellow black) file format. The known methods of translation can result in the flaws seen in FIG. 1, prior art.